Many applications for hydraulic fluids require that the viscosity of the fluid change as little as possible over the intended operating temperature range. Silicone fluid, and polydimethylsiloxane fluids in particular, are known to have a low viscosity variation with temperature relative to organic oils. This property, along with its thermal and oxidative stability, would thus be expected to make polydimethylsiloxane (PDMS) oil a highly desirable hydraulic fluid, and an example of this utility may be found in U.S. Pat. No. 2,398,187 to McGregor et al. However, in applications involving metal components which undergo relative motion, the utility of PDMS oil is largely constrained by the fact that it is a particularly poor metal lubricant (e.g., in steel-to-steel contact). This results in unacceptable wear of the metal surfaces, particularly at elevated temperatures and under high load conditions.
Efforts have been made to improve the lubricating properties of PDMS oils and the inclusion of various hydrocarbon oils and lubricity additives has been successful to some degree.
Morro et al., in U.S. Pat. No. 4,059,534, describe hydrocarbon/silicon (sic) oil lubricating compositions for low temperature use wherein polydimethylsiloxane is mixed with an alkene, isoparaffin or naphthenic oil.
In Canadian Patent Number 1,100,931 to Morro and Rathgeber, similar silicone/hydrocarbon blends are disclosed wherein a high viscosity polydimethylsiloxane is combined with a blend of a low viscosity polydimethylsiloxane and a hydrocarbon oil. These blends are said to have excellent viscosity-temperature characteristics and low temperature stability.
In another similar disclosure, U.S. Pat. No. 4,097,393 to Cupper et al. teaches lubricant and hydraulic fluid compositions consisting essentially of polydimethylsiloxane and particular hydrocarbon oils, wherein blends of these components remain miscible at -40.degree. C. for at least 72 hours.
However, certain hydraulic applications for fluids of the type described above (e.g., gas compressors, hydraulically actuated brakes and controls) are directed to systems wherein the temperature of the working fluid is maintained within a relatively narrow range. Under such conditions, a hydraulic fluid should have the lowest absolute viscosity consistent with other requirements of lubricity, stability and low temperature fluidity. Thus, a reduction in fluid viscosity translates directly into a reduction in the viscous power loss (i.e., cost, fuel conservation) associated with the operation of the hydraulic device. This reduction is particularly appreciated at low temperatures where the fluids exhibit significantly greater viscosities.